1. Field of the Invention
The present invention relates to a circuit module that handles a large amount of electric power.
2. Description of the Related Art
In recent years, environmental disruption, sign of global warming, and causes of the warming have been reported in newspapers. There exist a number of the causes, one of which is increased electric power consumption. The electricity largely depends on petroleum resources now being depleted, and a problem is emission of carbon dioxide gas into the atmosphere resulting from combustion of the petroleum. In addition, most of automobiles are gasoline-powered, which also worsen the problem.
Understandably, the former, electricity, is essential to drive electronic devices in every part of the world. Electricity serves as a power source for a washing machine, an air conditioner, portable devices, and the like, and is indispensable for all the people of the earth to maintain cultural life. Thus, the problem is a challenging theme to solve.
On the other hand, automobiles have become highly functional. For example, holding a TV conference within a vehicle or guiding to a destination by a car navigation system has become possible, a car air-conditioner is used for cooling, or head lights illuminate bright and sharply. As the functions become increasingly sophisticated, world's consumers prefer to buy these automobiles. In other words, unlike the old days, driving a car while using various functions in the car results in increased energy consumption. This also holds true for computers or portable phones.
In order to achieve these functions, semiconductor devices, such as so-called power devices, IC, and LSI have been introduced. These devices are mounted on a mounting board such as a printed circuit board, for example, and installed in a set of electronic devices. Given this matter, reduction in power consumption of semiconductor devices, has become an important issue.
The electronic devices, especially semiconductor devices, generate heat when operating, which increases the temperatures of their active areas, resulting in reduced driving ability. An attempt to enhance the driving ability leads to further increase in energy consumption.
In fact, it is necessary to reduce electric power that is consumed by semiconductor devices themselves by releasing heat of semiconductor devices to the outside in some way. The trend is clearly seen, for example, in power MOS devices that are capable of power driving, and requires some measure to release heat. For this reason, recently, devices such as inverter modules used in, for example, washing machines, refrigerators, and drive modules used in, for example, a plasma display have often been mounted on metal substrates to release heat.
The metal substrate has its surface covered by an insulating resin, and the like, and conductive patterns formed thereon. A device necessary for an inverter circuit, for example, is electrically connected to and mounted on the conductive patterns.
However, the number of circuit devices or conductive patterns that can be built on the top surface of one metal substrate is limited, and it was thus difficult to integrate on the top surface of one metal substrate a large hybrid integrated circuit formed of power devices and control devices. Then, forming conductive patterns in multiple layers on the top surface of a metal substrate would enable integration of a more complicated electric circuit. To make conductive patterns in multiple layers, however, an interlayer insulation layer should be provided between a lower layer conductive pattern and an upper layer conductive pattern, which may lead to a problem that the interlayer insulation film made of resin blocks conduction of heat released from circuit devices to the metal substrate.
In addition, if a metal substrate and a printed circuit board are superposed and arranged within casing member and connected with each other, a relatively large scale hybrid integrated circuit is achieved while utilizing an excellent heat release characteristic of the metal substrate. However, a circuit device made of a semiconductor material such as silicon, a metal substrate, and a printed circuit board have different thermal expansion coefficients, which thus results in problems that a solder connection between a circuit device and a printed circuit board may have cracks or conductive patterns formed on a metal substrate or a printed circuit board are disconnected.